1. Field of the Invention
The present invention relates to a process and apparatus for preparing a silicon oxide film, and more particularly to a method for preparing a silicon oxide film using an ECR plasma CVD apparatus comprising a plasma generation chamber which receives microwaves and a plasma raw material, an exciting solenoid which surrounds the plasma generation chamber concentrically and forms therein an electron cyclotron resonance magnetic field together with the microwaves, a plasma reaction chamber or processing chamber which receives a reactive gas and which communicates with the plasma generation chamber, and a substrate or specimen holder inside the plasma reaction chamber for holding a substrate in such a manner that a surface on which a film is to be grown faces the plasma generation chamber. The invention also relates to such an ECR plasma CVD apparatus, particularly for preparing a silicon oxide film.
2. Description of the Prior Art
Silicon oxide films, which are prepared mostly as interlayer insulator films in the process of fabricating semiconductor devices, are required to have various characteristics, such as film-forming properties at low temperatures, low internal stresses, low etching rates, good film thickness distributions, good step coverage, excellent waterproofing properties, etc.
Electron cyclotron resonance plasma CVD (hereafter, referred to as "ECR plasma CVD") has been proposed as a film-forming process which satisfies the aforementioned conditions. FIG. 1 is a schematic cross-sectional view showing the basic arrangement of a conventional ECR plasma CVD apparatus in accordance with the process.
As shown in FIG. 1, a conventional ECR plasma CVD apparatus 100 comprises a microwave generator 102 which is connected through a waveguide 104 to a plasma generation chamber 106 for generating plasma. The plasma generation chamber 106 has a first gas supply pipe 108 and a vacuum window (waveguide window) 110. Window 110 is disposed between the waveguide 104 and the plasma generation chamber 106 and gas-tightly separates the waveguide 104 (which is under atmospheric pressure) and the plasma generation chamber 106. Below the plasma generation chamber 106 is a metal plate 112 having a large diameter opening (plasma take-out window) 112A. The metal plate 112 and the plasma generation chamber 106 together define a half-opened microwave resonator. An excitation solenoid 114 surrounds the outer surface of the resonator in such a manner that a magnetic field adapted to satisfy the ECR conditions is generated, whereby plasma is produced within the resonator chamber. A plasma reaction chamber (processing chamber) 116 is arranged below the plasma generation chamber 106 and the metal plate 112. Inside plasma reaction chamber 116 is a substrate holder 118 which holds thereon a substrate 120. The plasma reaction chamber 116 has a second gas supply pipe 122 and an exhaust pipe 124 on its lower portion through which the reaction chamber 116 is connected to a vacuum system (not shown). A second excitation solenoid 126 is provided in coaxial relationship with the first excitation solenoid 114 and in a position sandwiching the substrate 120 in the axial direction together with the first excitation solenoid, i.e., on the rear side of the substrate. An RF power source 128 is connected to the substrate 120 through a line 130 insulated from the wall 116A of the plasma reaction chamber 116.
In the conventional arrangement shown in FIG. 1, the microwaves propagated within the waveguide 104 are introduced into the plasma generation chamber 106 through the waveguide window 110. Furthermore a magnetic field is formed within the plasma generation chamber 106 by means of the first excitation solenoid 114, so that the gas introduced from the first gas supply pipe 108 is converted into plasma making use of the electron cyclotron resonance phenomenon. The first excitation solenoid 114 creates a magnetic field which diverges toward the plasma reaction chamber 116, which communicates with the plasma generation chamber 106 through the plasma take-out window 112A. The diverging magnetic field causes the plasma generated in the plasma generation chamber 106 to be drawn out therefrom into the plasma reaction chamber 116. This plasma flow reaches the substrate 120 mounted on the substrate holder 118 while reacting with the gas introduced from the second gas supply pipe 122, thus forming a thin film on the substrate 120. Further, the second excitation solenoid (subsolenoid) 126 is arranged in coaxial relationship with the first solenoid 114 and in a position sandwiching the substrate 120 in the axial direction between the second excitation solenoid 126 and the first excitation solenoid 114. Current is applied to the first excitation solenoid 114 and the subsolenoid 126 in such a manner that the solenoids form magnetic fields in opposite directions so that both magnetic fields diverge or spread outward abruptly in the vicinity of the substrate. This creates a so-called cusp-shaped magnetic field 132 with a cusp plane 134 at a predetermined distance from the surface of the substrate 120. This cusp-shaped magnetic field ECR plasma CVD apparatus can form thin films with a uniform film thickness distribution and is being used more and more widely.
In the conventional process for forming thin films, a high density plasma can be obtained at low pressures within the range of 10.sup.-3 to 10.sup.-4 Torr, which makes it possible to form silicon oxide films having small internal stresses and high acid resistances without heating the substrate 120.
However, the conventional process for forming thin films as described above has various problems in step coverage, film thickness distribution, uniformity in the film thicknesses in stepped portions, performing the process at lower temperatures, etc., as summarized below.
(1) Step Coverage:
The conventional process has a disadvantage in that when a cusp-shaped magnetic field is not created, step coverage is insufficient, while in the presence of a cusp-shaped magnetic field other problems occur as will be described later on. In order to overcome the disadvantage in the absence of a cusp-shaped magnetic field, it has been proposed to apply high frequency power to the substrate in the case where there is a stepped portion such as wiring on the substrate to cover the stepped portion by means of a self-biasing effect. However, at a pressure within the range of 10.sup.-3 to 10.sup.-4 Torr, the application of high frequency power to the substrate results in the formation of an overetched portion in the next step in the fabrication of an LSI since the distribution of the HF etching rate in the substrate plane is as narrow as .+-.20%, thus decreasing the yield of LSI chip or giving rise to LSIs with low reliabilities.
(2) Film Thickness Distribution:
In the conventional process, if a cusp-shaped magnetic field is created in the vicinity of the substrate and high frequency power is applied to the substrate in order to obtain a uniform film thickness distribution, the film thickness distribution falls within the usually required range of .+-.5% as shown in FIG. 2 when the substrate has a diameter of 6 inches. If the substrate has a diameter of 8 inches, however, the film thickness distribution increases so as to fall within the range of .+-.10% or wider. Thus, the conventional approach for making the film thickness uniform cannot cope with an increase in the size of the substrate. FIG. 2 shows the results of an improvement in the film thickness distribution obtained by the application of high frequency power in addition to the creation of a cusp-shaped magnetic field. This gives rise to an electric field due to a negative floating potential appearing on the surface of the substrate based on the difference in mobility between electrons and ions, which electric field is stronger at the peripheral portion of the substrate than in the center thereof. Even with this approach, when the cusp plane is positioned at a distance of 50 mm from the rear side of the substrate, the film thickness distribution is as wide as .+-.20% as a result of the combined effect produced by the cusp-shaped magnetic field and the electric field on the surface of the substrate. Though not shown in FIG. 2, a ring-shaped magnetron resonance region, having a ring diameter proportional to the distance between the cusp plane and the substrate, is formed at the front side of the substrate when the cusp plane is positioned at the front side of the substrate, and the film thickness is larger in the ring region than in other regions.
As described above, in a low pressure region where the gas pressure is as low as 10.sup.-3 to 10.sup.-4 Torr, neither using a cusp-shaped magnetic field nor establishing a negative potential by applying high frequency power is enough to ensure uniformity of the film thickness over a broad region. If an attempt is made to improve the film thickness distribution by increasing the intensity of the cusp-shaped magnetic field (by increasing the current applied to the excitation solenoid 114 and the subsolenoid 126), there would arise a new problem as described in section (3) below, which would reduce the reliability of the resultant LSIs.
(3) Uniformity of Film Thickness of Stepped Portion:
FIG. 3 is a schematic cross sectional view showing the state of coverage of stepped portions when a thin film is formed in the presence of a cusp-shaped magnetic field in accordance with the conventional process.
Referring to FIGS. 1 and 3, when a cusp-shaped magnetic field is created in the vicinity of the substrate 120, ions in the plasma move under the influence of the magnetic field created by the excitation solenoid 114 and have inertia which is retained depending on the mean free path of the particles, with the result that the ions bombard the stepped portions too obliquely to give a symmetric step coverage. The asymmetry in step coverage becomes more pronounced toward the periphery of the substrate, and the substrate is covered as shown in FIG. 3 at its peripheral portion although some variation could be observed depending upon the position of the cusp plane. In the case of LSIs, the asymmetric step coverage results in defective insulation between the wiring patterns or insufficient dielectric strength of the insulators, thus reducing the reliability of LSIs.
More particularly, in an arrangement where an Si substrate 120 having a phosphate silica glass (PSG) film 130 thereon is provided with aluminum (Al) wiring patterns 132, magnetic force lines 134 are directed obliquely, i.e., not at right angles, to the substrate. Therefore, a thin film 136 formed on the substrate has a non-uniform film thickness dependent on the direction of the magnetic force lines.
(4) Lower Temperature Operation of the Process
Due to the aforementioned problems, it has conventionally been unsuccessful to apply a cusp-shaped magnetic field during film formation at a pressure of 10.sup.-3 to 10.sup.-4 Torr in order to form films in a diverging magnetic field. When the waterproofing properties or anti-water permeability of a film formed in a diverging magnetic field at a film formation temperature of 250.degree. C. or lower was examined, the anti-water permeability was found to be equal to or lower than that of a film obtained by the conventional CVD process without the diverging magnetic field. To solve this problem, the CVD process needs to be performed at a O.sub.2 /SiH.sub.4 ratio of 1.0.+-.0.2 and the gases needs to be supplied in stoichiometric amounts so that the chemical reaction can proceed ideally according to the following reaction scheme: EQU SiH.sub.4 +O.sub.2 .fwdarw.SiO.sub.2 +2H.sub.2
to reduce the takeup of O-H groups by excessive O.sub.2 or the amount of Si-H groups by excessive SiH.sub.4. With this countermeasure, good anti-water permeability has been obtained at a film formation temperature of 250.degree. C. or over.
However, these processes have narrow process margins or allowances with regard to the amounts of SiH.sub.4 and O.sub.2 that are supplied, and as a result minute changes in the control mechanism of the apparatus could decrease the anti-water permeability of the film.
The conventional processes have a problem in that at a temperature of 300.degree. C. or over, the underlying A1 wiring patterns are damaged and thus the probability that electromigration might occur increases by leaps and bounds (electromigration is a phenomenon in which intergranular boundaries in polycrystalline A1 grow at elevated film formation temperatures, and A1 atoms will move or migrate along the intergranular boundaries under application of current).